1. Field of the Invention
The present invention relates to a selective calling system, and more specifically to a format of a transmission signal, a method associated with transmitting and receiving the transmission signal and a receiver for receiving the transmission signal.
2. Related Art
As to a selective calling receiver which is usually taken on the road, the life of a battery as a power supply is a fundamental performance category. Especially, with the miniaturization of the receiver includes in recent years, a smaller battery is increasingly demanded. Therefore, how to make the battery life long becomes a more important technical problem.
The battery life is determined by two main parameters: the amount of the power consumption of the receiver and the intermittent receiving interval. Since a considerable portion of the power consumption is made in a radio frequency system, so far as a receiving operation is performed, one can not hope for a drastic improvement of the power consumption. Then, a method of making the intermittent receiving interval longer is proposed to reduce an average power consumption. In order to make the intermittent receiving interval longer, a synchronous system is under consideration in place of an asynchronous system (for example, POCSAG system) which is used widely at present. One example of the synchronous system is ERMES (European Radio Message System).
ERMES employs the following method. A frame number is given to each time division frame in advance and a base station transmits a calling signal with the frame number which a certain receiver should receive. This method allows the intermittent receiving interval to be lengthened dramatically as compared with the asynchronous system.
A transmitting signal of ERMES is comprised of a synchronization field, an address field and a message field. The message field is further comprised of a message and a message header indicating attributes of the message. Since the message header indicates which address the message belongs to, the transmission order of the message can be determined arbitrarily regardless of its address. For example, a paging system which is disclosed in Japanese Patent Unexamined Publication No. 63-158924 (publication date: Jul. 1st, 1988) employs a method for lengthening a receiving internal by designating the frame number which should be received.
However, the conventional selective calling system mentioned above needs a complicated system configuration. Further, a lot of additional bits are required in a transmission message signal, resulting in increased overhead of the message signal, which causes the message transmission efficiency to be reduced. Therefore, even though a high-speed transmission system is adopted, the expected increase of the subscriber capacity cannot be achieved.